where [1]}
\(P_e(ub) = \frac{1}{2}\Theta _{\Lambda }\left(\exp (-\frac{1}{8\sigma ^2})\right) - \frac{1}{2}=\frac{1}{2} \sum _{x \in \Lambda \setminus \lbrace  0 \rbrace  } \exp \left( -\frac{\Vert x\Vert ^2}{8\sigma ^2} \right).\) 
